Only five of New Zealand's 13 infant formula manufacturers have been approved to export product into China under new regulations that came into force today.

But Food Safety Minister Nikki Kaye says she is optimistic the remaining eight manufacturers will be able to meet the new rules shortly.

The new rules follow an audit of local baby milk manufacturing facilities that took place in March. Infant formula companies selling product in China must meet strict new requirements and become registered with the Chinese Government from today.

Fonterra, Auckland's Sutton Group and its joint venture partner Gardians, GMP Pharmaceuticals, Danone-owned Nutricia and the Waikato-based Dairy Goat Cooperative have been approved.

The Government says the five manufacturers are responsible for around 90 per cent of the roughly $200 million worth of infant formula New Zealand exports to China annually.

"Now we're working on getting the other eight other manufacturers across the line as quickly as possible."

A Fonterra spokesman said the dairy giant was one of the five companies that had achieved registration.

The Ministry's deputy director-general for China Relations, Roger Smith, told the Herald this week that companies that were not registered by May 1 (today) would be unable to export product manufactured after that date.

Kaye said China had indicated that other manufacturers could become registered after May 1 if they met the requirements verified by MPI.

"We feel very positively about the eight but obviously it will be up to them to determine that they can meet that overseas market access requirement that was issued last night."

When the manufacturers were in a position to demonstrate they net the new requirements, which for some could be a matter of days, " then we will immediately send in an MPI team to check verify compliance.

"We will then immediately send that documentation to China and we hope they will then registered."

That would be done on an individual manufacturer basis rather than waiting for the full eight which would speed up the process.

"There is a team of people at MPI who will be assisting the remaining manufacturers to be registered as soon as possible" Kaye said.

Kaye said last week that around 50 infant formula companies that do not operate factories but instead have their products made by contract manufacturers would find it more difficult to comply with the new regulations.

Today, she said some brand owners who did not have a "close relationship" with their manufacturer would struggle to meet the new rules.

Guy said that meant "there will be some thinning out of brands".

"MPI has been working with industry and Chinese agencies to give brand owners greater clarity on what closer association will mean and when this will apply," Kaye said.

"That means that they have to have great oversight from the brand right through to the manufacturer", Guy said.

The new rules and registration requirement apply to all countries exporting infant formula to China.